U.S. Pat. No. 4,044,521 shows a roof structure including a plurality of spaced, parallel I-beams, a plurality of spaced, parallel purlins of generally Z-shaped cross section supported by and extending crosswise of the I-beams, roof panels on top of the purlins, extruded plastic roof liner support members hooked over the lower free end portions of the purlins, corrugated roof liner panels supported at opposite edges respectively on roof liner support members on pairs of adjacent spaced purlins, and insulation batts between the roof liner panels and the roof panels on top of the purlins. Fibrous glass boards, faced with impervious sheet material to provide a vapor seal, have been used in place of the corrugated roof liner panels.